A man fatally stabbed his 17-year-old sister, decapitated his five-year-old sister in front of a police officer and then turned toward his nine-year-old sister with a knife in his hand before officers shot him dead in what their chief described as "a killing field".

There was no clear motive for the events that unfolded on Saturday, the day after the five-year-old’s birthday, in Boston, USA. But there was no doubt about the carnage wrought by 23-year-old Kerby Revelus against his sisters in the two-family home they shared with their parents and grandmother.

Five-year-old Bianca was killed as a cake for her birthday, which investigators believe was Friday, sat on the kitchen table. Nine-year-old Sarafina dialed 911 and watched police shoot her brother as her elder sister, 17-year-old Samantha, lay dead on the floor.

Sarafina was hospitalised Sunday with defensive wounds to her hands and stab wounds in her abdomen and one of her legs, police said.

"In policing, we see the raw human emotion every day, but to think that a human being could afflict such an atrocious, violent act on his own family is unbelievable," Milton police Chief Richard G. Wells Jr. said. "When I walked up to the first officer (on the scene), I could see the whole story right in his face. This just told me that this was something very bad."

Saturday’s attack came about 24 hours after Revelus had gotten into a fistfight with a man living next door, Wells said.

"Blows were exchanged," he said. "I don’t know the cause of it, but we’re confident that did happen. He had been agitated in the hours that followed that, going into the day and last night."

Investigators believe Revelus targeted Samantha, a senior at Milton High School, and fatally stabbed her with a household knife while their grandmother, who neighbors say lives on the first floor, was doing laundry in the basement. The children’s parents were away; their mother, identified by friends as Regine Revelus, is a nurse at a Boston hospital, Wells said.

Sarafina, a student at the Tucker Elementary School, just behind the house, called 911 around 5 p.m. An officer on patrol in the neighborhood arrived within a minute, Wells said, and could hear an altercation inside as he reached the second floor. The 911 operator tried to persuade Sarafina to open the door, but when she didn’t the officer broke through.

"As the officer entered the door, (Revelus) decapitated (Bianca) in front of him," Wells said. "He actually walked into a killing field. He walked into such carnage, as far as the atrocity of it, I’ve never seen it."

Within moments, four officers were inside and two of them shot Revelus as he tried to get to Sarafina, Wells said. Revelus fell, still clutching the knife.

Details about the number of shots and who killed Revelus were pending the outcome of an autopsy on Sunday.

Revelus had recently served jail time on a gun charge, Wells said, but the details would not be released until courts opened Monday. Neighbors said Revelus was in a car that was pulled over by police and from which one occupant threw a gun into a sewer.

Police had been called to the family’s house in 2004 after a domestic violence report that Revelus had punched a woman living there, Wells said.

A neighbor, Norm Walsh, said his daughter Kate Walsh, a Hollis, N.H., police officer, reported hearing two shots as she went outside to bring in groceries from the family car. Moments later, a blood-covered officer emerged carrying Sarafina, seeking towels to stanch her bleeding.

"It’s shocking to me," said Norm Walsh, whose son is the same age as Revelus. "He played a lot of pickup hoop in the driveway."

Walsh said the family members had lived in the neighborhood for more than 20 years and were warm. They were of Haitian descent, as are many residents in the neighborhood, where Creole is spoken alongside English.

"The family is a solid family. Both parents worked; good kids. Completely makes no sense," Walsh said.

Samantha’s classmates referred to her by her nickname, Princess, and remarked at her grace, class and friendliness. She had been one of about 20 students who had been at the high school Saturday afternoon to rehearse for a poetry jam on Thursday. Saturday morning, Samantha also had practiced for a school fashion show.

"She had a stage presence like you couldn’t believe," classmate Kassi Stein said.

Sobbing, she added: "She had just a soft voice and everyone would lean in to hear what she was saying."

In one of the poems Samantha was to recite, "Acquaintance," she wrote of the strength of women in the face of "ignorant souls."

The poem closed, "So what lousy wind brought you here? What values you offer? She’s a woman, a queen, a goddess. Don’t treat her like any other."

A two-hour grief counseling session was held at the school Sunday afternoon and will be offered Monday to students and employees.

The officers involved in the case were placed on administrative leave and were receiving counseling from the Boston Police Department.